The invention relates to gas turbine engines. More particularly, the invention relates to alloy selection and crystalline orientation for gas turbine engine components.
Gas turbine engine components are commonly formed of alloys, typically nickel- or cobalt-based superalloys. Many components, such as blades are formed of single crystal (SX) alloys. In such single crystal components, essentially the entire component is formed of a single continuous crystal lattice. Typically, the orientation of that lattice is predetermined to achieve desired properties of the component. The orientation may be assured by use of a grain starter or other casting techniques.
By way of example, U.S. Pat. No. 4,605,452 of Gemma and Dierberger identifies face centered cubic (FCC) single crystal gas turbine blades wherein the [110] crystal axis is tangent to the blade airfoil surface in a critical crack-prone region just behind the leading edge of the airfoil at about 40-80% of the airfoil span.
Generally most materials subjected to elastic tensile stress shrink transversely to the direction in which the tension is applied. Similarly, they will transversely expand if compressed. This is called Poisson's effect and it is characterized as a negative ratio of the transverse strain to the longitudinal strain. Typically the value of Poisson's ratio is around +0.33 for most isotropic (i.e., properties identical in all directions) engineering structural materials. FIG. 1 shows a tensile stress σ (sigma) applied to a piece 20 of such material. The material is strained to a condition 20′ wherein it has expanded along the direction of the stress and contracted transverse thereto.
Several naturally occurring materials such as cork and some artificially made foams are known to display negative Poisson's ratio. Such materials transversely expand when pulled and shrink when compressed. Such materials are often referred to as auxetics. Auxetic materials such as cork make good sealing material and find many unusual applications. For example, cork makes a good seal plug for wine bottles. When pushed at an end, the cork does not substantially expand diametrically. This facilitates ease of insertion through the bottle mouth and provision of a good seal. FIG. 2 shows a tensile stress a (sigma) applied to a piece 22 of such material. The material is strained to a condition 22′ wherein it has expanded along the direction of the stress and also expanded in at least one direction transverse thereto.